What is Toxoplasmosis?

Toxoplasmosis is a disease caused by infection with the organism called Toxoplasma gondii (T. gondii). This is a microscopic single-cell protozoal organism related to coccidia. Virtually all warm-blooded animals, including people, can be infected with this organism. It is an extremely well adapted parasite and rarely causes significant disease to the individuals it infects. Despite the high prevalence of infection, the parasite rarely causes significant disease in cats – or in any other species.

How Common is Toxoplasma in Cats?

Toxoplasma occurs worldwide and infection in cats is similarly widespread. Many more cats are infected than show symptoms. In research studies, as many as one-half of adult cats in certain geographical areas have antibodies to the organism in their blood, indicating that they have been exposed to the infection at some time. Infection rates are higher in free-roaming and stray cats. In contrast, infection is uncommon in pet cats that do little or no hunting, and eat commercial cat foods primarily or exclusively.

How is Toxoplasma Transmitted?

Cats are usually infected by ingesting the organism present in the tissues (meat) of another infected animal, known as an ‘intermediate host’. The intermediate host is usually a rodent. After infection, the Toxoplasma organism replicates or reproduces locally in the intestinal tract of the cat, and is usually contained there. The replication in the intestinal tract results in shedding of oocysts (a form of eggs) in the feces. Oocysts represent a hardy form of the organism that can survive in the external environment for many months or even years. Other animals can become infected by ingesting these oocysts, but disease will result only if large numbers are ingested.

How Do People Get Toxoplasmosis?

While cats are usually infected by eating infected rodents, or more rarely by ingestion of oocysts from the environment, humans are most commonly infected by eating contaminated food. Sheep, cattle and pigs grazing on contaminated pastures, or fed oocyst-contaminated food, can develop the encysted form of the organism in their body tissues. If infected meat (typically beef, pork or lamb) is not adequately cooked, or if proper hygiene precautions are not followed during handling of uncooked meat, humans can become infected.

Ingestion of oocysts from infected cats, for example during gardening in contaminated soil, is a much less common source of human infection. Because cats only shed the organism for a few days in their entire life, the chance of human exposure is relatively small. Owning a cat does not mean you will be infected with toxoplasma. It is unlikely that you would be exposed to the parasite by touching an infected cat, because cats usually do not carry the parasite on their fur. It is also unlikely that you can become infected through cat bites or scratches. In addition, cats kept indoors that do not hunt prey or are not fed raw meat are not likely to be infected with T. gondii.

How Important is Toxoplasma in People?

As with infection in cats, the vast majority of people infected with this organism do not develop clinical disease at all, or possibly just show mild and transient flu-like signs. However, there are some cases in which significant disease does occur and one situation is particularly important. If a pregnant woman acquires Toxoplasma infection during her pregnancy, the infection may be transmitted to the fetus, and sometimes causes severe damage.

This is only a risk if the woman acquires the infection during her pregnancy. A woman who has previously been exposed to the organism caries no risk of transmission to her fetus if she subsequently becomes pregnant.

Are Cat Owners More Likely to Develop an Infection?

Although cats are essential to complete the life-cycle of T. gondii, numerous surveys have shown that people who own cats have no higher a risk of acquiring infection than people who don’t own cats. There are several reasons for this:

• Many pet cats will never be exposed to Toxoplasma and therefore cannot pass infection on to humans.
• Even if a cat does become infected with Toxoplasma, it will only shed the oocysts or eggs in its feces for approximately ten days after initial exposure. Following this there is no further significant oocyst shedding and therefore again no further risk to humans.
• Although humans may become infected through exposure to oocysts in the environment, a more common source of infection appears to be infected meat.

How Can Humans Avoid Infection?

The Centers for Disease Control and Prevention (CDC) recommend the following to prevent contracting toxoplasmosis:

• Do not eat raw or under cooked meat. Meat should be cooked to a temperature of at least 160°F for 20 minutes.
• Do not drink unpasteurized milk.
• Do not eat unwashed fruits and vegetables.
• Wash hands and food preparation surfaces with warm soapy water after handling raw meat.
• Wear gloves when gardening. Wash hands after gardening.
• Wash hands before eating (especially for children).
• Keep children’s sandboxes covered.
• Do not drink water from the environment unless it is boiled.
• Feed cats only canned or dried commercial food or well-cooked table food, not raw or undercooked meats. Also, do not give them unpasteurized milk.
• Do not allow cats to hunt or roam.
• Do not allow cats to use a garden or children’s play area as their litter box.
• Remove feces from the litter box daily and clean with boiling or scalding water.
• Pregnant women, and persons with suppressed immune systems, should not clean the litter box.
• Do not adopt or handle stray cats, especially kittens. Do not get a new cat while you are pregnant.
• Control rodent populations and other potential intermediate hosts.